1. Field of the Invention
This invention relates generally to a bicycle multiple chainwheel which includes at least two diametrically different sprockets. More particularly, the present invention relates to a bicycle multiple chainwheel which is improved to provide reliable chain shifting from a larger sprocket to a smaller sprocket.
2. Description of the Prior Art
As is well known, a bicycle is driven by a combination of a front gear and a rear gear connected thereto by a chain. The front gear is normally called "chainwheel", whereas the rear gear is usually referred to as "freewheel". The chainwheel is mounted on a pedalling crank assembly, and a forward pedalling force is transmitted to the freewheel through the chain for driving the rear wheel.
In such a bicycle, when the freewheel includes two or more diametrically different sprockets, it is possible to provide two or more speeds by shifting the chain from one sprocket to another. Further, if the chainwheel also includes two or more diametrically different sprockets, it becomes possible to provide an additionally increased number of speeds. For example, when the chainwheel consists of three sprockets with the freewheel comprising five sprockets, the cyclist can select 15 speeds in total.
The multiple chainwheel comprises a larger sprocket located at a laterally outermost position relative to the bicycle frame, and at least one smaller sprocket arranged laterally inwardly of the larger sprocket. In such a chainwheel, the larger sprocket provides a higher speed, as opposed to the multiple freewheel wherein a larger sprocket provides a lower speed. For conducting a speed change at the chainwheel, use is made of a front derailleur which is located adjacent to the chainwheel in a forwardly moving path of the chain. When the front derailleur is laterally moved relative to the chainwheel, the chain is laterally pressed to shift from the larger sprocket to the smaller sprocket or vice versa.
To explain the disadvantages of a prior art multiple chainwheel, reference is now made to FIGS. 7 and 8 of the accompanying drawings which show a typical multiple chainwheel.
As shown in FIGS. 7 and 8, the prior art multiple chainwheel CW' comprises a larger sprocket 1', and a smaller sprocket 2' coaxial with the larger sprocket. The larger sprocket usually has a plurality of equiangularly spaced openings 3' which provide weight reduction of the chainwheel as a whole in addition to increasing visual attractiveness. The weight reduction openings should be as large as possible on the condition that the larger sprocket can meet strength requirements. Thus, each weight reduction opening 3' should have a radially outer margin 31 located as maximally close to the outer circumference of the larger sprocket 1'. The smaller sprocket 2' may similarly have weight reduction openings.
With the arrangement described above, when there is a great diametrical difference between the two sprockets 1', 2', the outer margin 31 of each weight reduction opening 3' of the larger sprocket is located considerably away from the outer circumference of the smaller sprocket, forming a relatively large radial clearance H between the opening outer margin 31 and the outer circumference of the smaller sprocket. This radial clearance may give rise to a serious problem in conducting speed change, as described below.
When shifting a chain C' (see FIG. 8) from the larger sprocket 1' to the smaller sprocket 2', the chain is laterally pressed inward by an unillustrated front derailleur, causing the chain to start disengaging from the larger sprocket from a specific circumferential point thereof. Since the chain is always subjected to driving tension, the thus disengaged chain extends substantially horizontal for shifted tangential engagement with the smaller sprocket 2', as shown in FIG. 8.
If the large radial clearance H at any weight reduction opening 3' of the larger sprocket coincides in position with the horizontally disengaging chain C', the chain may move laterally into that particular clearance or opening. As a result, the chain may fall between the two sprockets, and get firmly trapped there by large tension applied to the chain. Upon further rotation of the chainwheel, the chain thus unnaturally trapped may damage the chainwheel or the front derailleur.
One conceivable way to solve the above problem is to reduce the size of each weight reduction opening 3' of the larger sprocket 1' to such a degree that no radial clearance H is formed. However, this solution is not readily applicable because of unacceptable weight increase and appearance deterioration.
Another possible solution to solve the problem of the prior art is to reduce axial or lateral spacing between the two sprockets 1', 2' to the degree of prohibiting chain trapping therebetween. However, this solution gives rise to a new problem that the chain is likely to be overshifted beyond the smaller sprocket, thus completely coming out of driving engagement with the chainwheel.